The study investigated the effects of cooperative learning on junior high school students who worked in structured or unstructured cooperative groups. Two hundred and twenty-three junior high school students participated in the study and worked in three or four-person, mixed gender and achievement groups. The results show that the children in the structured groups were more willing to work with others on the assigned tasks and they provided more elaborate help and assistance to each other than their peers in the unstructured groups. Furthermore, as the children in the structured groups had more opportunities to work together, they developed a stronger perception of group cohesion and social responsibility for each other’s learning than their peers in the unstructured groups.